Why I receive a NullPointerException error? Line 52

The class should take the image and in the black pixels draw a rect of random color.

import processing.video.*;

Capture realTime;
ritrattoClass selfie;
PImage img;

void setup()
{
   size(640,480);
   background(0);
   textSize(40);
   text("Say Cheese and Press the Mouse", 5, width/3);
   realTime= new Capture(this,640,480);
   realTime.start();
   loadPixels();
}

void captureEvent(Capture realTime)
{
  realTime.read();
}

void draw()
{
  //image(realTime,0,0);
  if (mousePressed == true) {
    realTime.stop();
    img.filter(THRESHOLD);
    image(img,0,0);
    save("selfie.jpg");
    selfie=new ritrattoClass(img);
    selfie.ritratto();
  }
}

void mousePressed() {
  img=realTime.copy(); //saveFrame("selfie.jpg");
}

-----------Class-------------

class ritrattoClass{

  PImage img;
  PImage foto;

  ritrattoClass(PImage foto){
    img=foto;} //nel programma foto sarà una variabile con l'immagine "selfie"


  void ritratto(){
      foto.loadPixels();
      for (int y=0; y<foto.height; y+=3)
      {
       for (int x=0; x<foto.width; x+=3)
       {
         int index = x+foto.width*y;
         color col= foto.pixels[index];
         if (col==0){
         fill(random(255),random(255),random(255));
         rect(x,y,3,3);
         }
       }
      }
    updatePixels();
  }
}